Support before_reset callback in CurrentAttributes

This is useful when we need to do some work associated to `Current.reset`
but that work depends on the values of the current attributes themselves.
This cannot be done in the supported `resets` callback because when the
block is executed, CurrentAttributes's instance has already been reset.

For symmetry, `after_reset` is defined as alias of `resets`.

activesupport/CHANGELOG.md

+*   Add `before_reset` callback to `CurrentAttributes` and define `after_reset` as an alias of `resets` for symmetry.
+
+    *Rosa Gutierrez*
+
 *   Add `ActiveSupport::HashWithIndifferentAccess#assoc`.
 
     `assoc` can now be called with either a string or a symbol.
 
     *Stefan Schüßler*
 
-
 ## Rails 6.0.0.beta1 (January 18, 2019) ##
 
 *   Remove deprecated `Module#reachable?` method.

activesupport/lib/active_support/current_attributes.rb

@@ -119,10 +119,16 @@ def attribute(*names)
         end
       end
 
+      # Calls this block before #reset is called on the instance. Used for resetting external collaborators that depend on current values.
+      def before_reset(&block)
+        set_callback :reset, :before, &block
+      end
+
       # Calls this block after #reset is called on the instance. Used for resetting external collaborators, like Time.zone.
       def resets(&block)
         set_callback :reset, :after, &block
       end
+      alias_method :after_reset, :resets
 
       delegate :set, :reset, to: :instance
 

activesupport/test/current_attributes_test.rb

@@ -3,13 +3,18 @@
 require "abstract_unit"
 
 class CurrentAttributesTest < ActiveSupport::TestCase
-  Person = Struct.new(:name, :time_zone)
+  Person = Struct.new(:id, :name, :time_zone)
 
   class Current < ActiveSupport::CurrentAttributes
     attribute :world, :account, :person, :request
     delegate :time_zone, to: :person
 
-    resets { Time.zone = "UTC" }
+    before_reset { Session.previous = person.try(:id) }
+
+    resets do
+      Time.zone = "UTC"
+      Session.current = nil
+    end
 
     def account=(account)
       super
@@ -19,6 +24,7 @@ def account=(account)
     def person=(person)
       super
       Time.zone = person.try(:time_zone)
+      Session.current = person.try(:id)
     end
 
     def request
@@ -30,9 +36,14 @@ def intro
     end
   end
 
+  class Session < ActiveSupport::CurrentAttributes
+    attribute :current, :previous
+  end
+
   setup do
     @original_time_zone = Time.zone
     Current.reset
+    Session.reset
   end
 
   teardown do
@@ -56,16 +67,28 @@ def intro
   end
 
   test "set auxiliary class via overwritten method" do
-    Current.person = Person.new("David", "Central Time (US & Canada)")
+    Current.person = Person.new(42, "David", "Central Time (US & Canada)")
     assert_equal "Central Time (US & Canada)", Time.zone.name
+    assert_equal 42, Session.current
   end
 
-  test "resets auxiliary class via callback" do
-    Current.person = Person.new("David", "Central Time (US & Canada)")
+  test "resets auxiliary classes via callback" do
+    Current.person = Person.new(42, "David", "Central Time (US & Canada)")
     assert_equal "Central Time (US & Canada)", Time.zone.name
 
     Current.reset
     assert_equal "UTC", Time.zone.name
+    assert_nil Session.current
+  end
+
+  test "set auxiliary class based on current attributes via before callback" do
+    Current.person = Person.new(42, "David", "Central Time (US & Canada)")
+    assert_nil Session.previous
+    assert_equal 42, Session.current
+
+    Current.reset
+    assert_equal 42, Session.previous
+    assert_nil Session.current
   end
 
   test "set attribute only via scope" do
@@ -92,13 +115,13 @@ def intro
   end
 
   test "delegation" do
-    Current.person = Person.new("David", "Central Time (US & Canada)")
+    Current.person = Person.new(42, "David", "Central Time (US & Canada)")
     assert_equal "Central Time (US & Canada)", Current.time_zone
     assert_equal "Central Time (US & Canada)", Current.instance.time_zone
   end
 
   test "all methods forward to the instance" do
-    Current.person = Person.new("David", "Central Time (US & Canada)")
+    Current.person = Person.new(42, "David", "Central Time (US & Canada)")
     assert_equal "David, in Central Time (US & Canada)", Current.intro
     assert_equal "David, in Central Time (US & Canada)", Current.instance.intro
   end